Support for custom interpolations when generating the outgoing email

The `QueuedMail` class currently provides a few fixed interpolations such as `%{email}`, `%{email_key}`, `%{campaign}`, etc.

It would make sense to be able to allow custom interpolations in some way which would not need changing the gem itself. Maybe something which receives a `QueuedMail` instance and uses that to provide the custom interpolations.
